In the last stage of a low pressure turbine or in a stage one or two stages before the last stage, the pressure is typically very low. Accordingly, the working fluid is in a wet steam state containing liquefied fine droplets (liquid droplet nuclei). The liquid droplet nuclei that are condensed and attached to the blade surface are combined together, so as to form a liquid film on the blade surface. Further, the liquid film is torn apart due to the mainstream steam and is sprayed into the downstream as coarse droplets that are far larger than the liquid droplet nuclei at the beginning. Although these coarse droplets are slightly scaled down by the mainstream steam afterward, the coarse droplets flow down while keeping certain sizes. The coarse droplets cannot rapidly turn along the flow passage like a steam due to their inertial forces, and collide with the moving blade in the downstream at high speed. This causes erosion in which the blade surface is eroded or causes a loss due to interference of the rotation of the turbine blade. In contrast, conventionally, in order to prevent an erosive action by an erosion phenomenon, the tip portion of the moving-blade leading edge is coated with a shield material. The shield material is made of a material that is hard and has a high strength, for example, stellite. Alternatively, there is a method in which various unevenness processing is performed on the surface of the leading edge portion of the blade to form a rough surface so as to reduce the impact force during collision of the droplets. However, the shield material cannot always be disposed due to the processability. Since only protecting the blade surface is not generally perfect as an erosion countermeasure, another method of the erosion countermeasure is usually used in combination.
Generally, to reduce the influence of the erosion, it is most effective to remove the droplets themselves. For example, Patent Literature 1 (JP-A No. Hei 1-110812) discloses a method to remove the droplets. In the method, the stationary blade employs a hollow stationary turbine blade and slits are disposed on the blade surface. By reducing the pressure inside of the hollow stationary turbine blade, the liquid film is suctioned. These slits are often directly processed on the blade surface of a stationary-blade structure with a hollow structure. Additionally, as disclosed in Patent Literature 2 (JP-A No. 2007-23895), there is a method for processing a slit portion as a separate member to mount the slit portion on the stationary blade. Additionally, Patent Literature 3 (JP-A No. Hei 8-240104) discloses a method in which a porous cover is disposed at an opening formed in the guide vane of the steam turbine, all capillaries of the porous cover are filled with liquid to be suctioned, and a wall withstanding the application of the negative pressure is formed by the porous cover and the capillaries filled with liquid such that the liquid penetrates the portion wet with the liquid on the wall.